The present invention relates to a VTR (video tape recorder), and more particularly, to automatic tracking arrangements for such a VTR.
In helical scan VTRs a magnetic tape having recorded tracks is displaced around a drum. Within the drum is a headwheel containing magnetic reproducing heads. The reproducing heads must "track" (stay aligned with) the recorded tracks in order to obtain sufficient amplitude for proper reproduction of the recorded signal. However, without some kind of tracking system, tracking errors occur. Most common types of errors for helical scan VTRs are related to the rotation (scanning) rate of the headwheel, which is 60 Hz for an NTSC recorded signal. In particular, tracking errors occur at said rotation rate and harmonics thereof, e.g. 60 Hz, 120 Hz, etc., with 360 Hz being about the highest harmonic at which significant errors occur.
A prior art method for controlling tracking errors has the reproducing heads mounted on a bimorph, which in turn is attached to the headwheel, and an oscillating "dither" signal applied to the bimorph to "dither" the heads. If a tracking error occurs, phase and amplitude changes in the reproduced signal are produced, which changes can be detected to provide information as to the direction and magnitude of the error. From this information a tracking control signal is generated and applied to the bimorph to ensure tracking.
In accordance with the Nyquist sampling criteria, the frequency of the dither signal must be at least twice the highest significant tracking error frequency, i.e. 720 Hz, if the highest significant error frequency is 360 Hz. Preferably the dither frequency is about three times said error frequency, i.e. 1080 Hz in the assumed example. However, this is in the range of the mechanical resonance frequency of the heads and associated mountings, which typically is from 500 Hz to 1200 Hz. Thus the dither signal may cause uncontrolled head vibrations, thereby making controlled tracking difficult.
An additional disadvantage of the dither technique is that in the normal tracking mode, the playback head at least partially leaves the recorded track, thereby periodically reducing the carrier signal level and decreasing the carrier-to-noise ratio.
It is therefore desirable to provide a highly controllable tracking system, in particular one that does not make use of a dither signal.